Method and system for providing time offset to minislot clock and count in headend devices

ABSTRACT

A method and system for allocating an initial maintenance request (IMR) for an upstream channel in a communications system, wherein the communication system includes a headend and at least one remote device associated with the channel. A first propagation delay from the headend to the remote device having the greatest delay is determined. Likewise, a second propagation delay from the headend to the remote device experiencing the least delay is determined. The IMR is then defined to be shorter than the first propagation delay and at least as long as the difference between the two propagation delays. The starting point of the IMR is established by modifying the clock output of the headend. A modification value is added to the headend clock output. The modification value corresponds to a time interval that can be as long as the propagation delay from the headend to the remote having the shortest delay.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/261,270, filed Jan. 12, 2001, incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention described herein relates to time division multiple access(TDMA) communications, and more particularly to synchronization betweena headend and remote devices across a TDMA system.

2. Background Art

Certain communication systems include a set of remote communicationsdevices connected to a headend device, such that the headend isresponsible for distribution of information content to the remotes. Insuch a system, the headend may also have administrative functions, suchas management of communications between the headend and the remotes.Transmissions from the headend to one or more remotes are denoted hereinas downstream transmissions. Transmissions in the opposite direction,from a remote to its associated headend, are denoted herein as upstreamtransmissions. Because there can be several remotes associated with asingle headend, upstream communications must be administered so as tomaintain order and efficiency. An adequate level of service needs to bemaintained. This can be done through the use of multiple channels in theupstream direction, and the use of time division multiple access (TDMA)communications in each channel of the upstream. In such an arrangement,the upstream bandwidth for each channel is controlled and allocated bythe headend. Any given remote can transmit upstream only afterrequesting bandwidth and receiving a grant of the bandwidth from theheadend.

One standard by which such a communications system can operate is theData Over Cable System Interface Specification (DOCSIS). DOCSIS wasoriginally conceived for cable communications systems. While DOCSIS canbe applied to such communications systems, it is not necessarily limitedto cable. Wireless communications systems, for example, can also operateunder DOCSIS. Likewise, DOCSIS can be used in satellite communicationssystems.

In the realm of cable communications, DOCSIS specifies the requirementsand objectives for a cable headend and for remote cable modems. A cableheadend is also known as a cable modem termination system (CMTS). DOCSISconsists of a group of specifications that cover operations supportsystems, management, data interfaces, as well as network layer, datalink layer, and physical layer transport. Note that DOCSIS does notspecify an application layer. The DOCSIS specification includesextensive media access layer (MAC) and physical (PHY) layer upstreamparameter control for robustness and adaptability. DOCSIS also provideslink layer security with authentication. This prevents theft of serviceand provides some assurance of traffic integrity.

The current version of DOCSIS (DOCSIS 1.1) uses a request/grantmechanism for allowing remote devices (such as cable modems) to accessupstream bandwidth. DOCSIS 1.1 also allows the provision of differentservices to different parties who may be tied to a single modem. Withrespect to the processing of packets, DOCSIS 1.1 allows segmentation oflarge packets which simplifies bandwidth allocation. DOCSIS 1.1 alsoallows for the combining of multiple small packets to increasethroughput as necessary. Security features are present through thespecification of 56-bit data encryption standard (DES), encryption anddecryption to secure the privacy of a connection. DOCSIS 1.1 alsoprovides for payload header suppression, whereby unnecessary ethernet/IPheader information can be suppressed for improved bandwidth utilization.DOCSIS 1.1 also supports dynamic channel change. The downstream channelor the upstream channel or both can be changed on the fly. This allowsfor load balancing of channels and can improve robustness.

In communications systems such as this, propagation delay can be anoperational concern and must be accommodated. Any transmission, upstreamor downstream, between a headend and a remote device will require someamount of time to reach its destination. Moreover, given a headend andseveral associated remotes, the upstream propagation delay between eachof the remotes and the headend may be different. Efficient upstreamcommunication, however, requires synchronization between a headend andeach of its remotes. Contention is minimized and processing is made moreefficient if, for example, a headend knows when to expect a transmissionfrom a remote. This is possible only when the headend and each remotehave the same sense of time.

DOCSIS provides a solution to the upstream synchronization problem. Theheadend sends out a synchronization message to all remote devicesassociated with the headend. The synchronization message contains a32-bit time stamp, based on 10.24 megahertz (MHz) clock. The time stampis a statement of the value of the headend's clock at the time oftransmission of the synchronization message. The time stamp is used toachieve synchronicity with respect to the upstream communications, byproviding each remote with the clock value of the headend, current as ofthe time of transmission of the synchronization message. Each remotedevice then locks the frequency and phase of its local clock counter tomatch the count contained in the received time stamp.

Note that the 10.24 MHz clock can, in some systems, be interpreted interms of time units, or “ticks.” Each tick can, for example, be 6.25microseconds. Ticks can be further organized into larger units calledminislots. The number of ticks per minislot can be defined at thediscretion of the headend. The available upstream bandwidth cantherefore be viewed as a series of minislots.

After receiving the time stamp, each remote then adjusts its local clockto compensate for some of the propagation delay between it and theheadend. This compensation step takes into account the known factorsthat contribute to overall propagation delay. Such factors includesystem topology and downstream interleaving. This compensation is knownas ranging offset. Each remote adds the ranging offset to its local32-bit clock The resulting clock value is then arithmetically convertedinto a minislot count.

After the synchronization message, the headend sends an initial mapmessage (commonly denoted in its capitalized form, “MAP message” andused hereinafter in this form) to all its remote devices operating on agiven channel. This message, in general, tells each remote whatminislot(s) the remote can use for transmission in the upstream. Thismessage therefore maps remotes to minislots. This message also defines aspecific point in the upstream (e.g., a specific minislot) at whichremotes are to respond. When a remote's response is received at theheadend, the headend compares the actual arrival time in the upstreamwith the expected arrival time. Any difference between these two pointsrepresents additional (as yet unaccounted for) propagation delay withrespect to the responding remote. The headend can then inform the remoteof this difference, allowing the remote to further adjust its localclock. As a result of this adjustment, the headend and the remote willhave the same sense of time with respect to upstream communications.

In particular, the initial MAP message defines, in the upstream, astarting point (a minislot) in an initial maintenance region (IMR). TheIMR represents an interval in the upstream during which any of theassociated remotes operating on the given channel can respond to theheadend. The MAP message therefore allocates upstream bandwidth. Becausethe MAP message defines the initial point in time (minislot) in theupstream at which a remote can respond, each remote will respond whenits minislot count corresponds to the minislot identified in the MAPmessage. The headend will then expect a response at that point in timein the upstream.

However, there will typically be some residual propagation delay. Theheadend will expect a response at a certain point in time in theupstream; the remote will transmit a response at what it believes to bethat point in time in the upstream. When the transmission arrives at theheadend, it will typically be somewhat later than expected by theheadend. This delay represents residual propagation delay of theresponding remote. The headend will then tell the remote the size ofthis residual propagation delay. This allows the remote to furtheradjust its internal time stamp (TS) counter by this amount. At thispoint, the remote is effectively synchronized with the headend. Asubsequent message sent by the remote at a specific point in time in theupstream will therefore be received at the headend at what the headendunderstands to be that point in time in the upstream.

Note that the IMR, as allocated by the headend, must be sufficientlylarge to accommodate any possible propagation delay. As a result, theIMR might represent a significant amount of time (i.e., bandwidth) inthe upstream. The IMR must accommodate all of the possible propagationdelays for the set of remotes associated with a headend with respect toa particular upstream channel. In some communications systems, however,upstream bandwidth is valuable. It represents opportunities for remotesto transmit information back to the headend. Such transmissions canrepresent sources of revenue for a communications system provider.Therefore, dedicating a substantial IMR for purposes of achievingsynchronization throughout the system represents an inefficiency and apossible loss of revenue to the system provider. Therefore, there is aneed for a synchronization process that requires less of the upstreambandwidth for an IMR.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method and system for allocating an initialmaintenance request (IMR) for an upstream channel in a communicationssystem, wherein the communication system includes a headend and at leastone remote device associated with the channel. A first propagation delayfrom the headend to the remote device having the greatest delay isdetermined. Likewise, a second propagation delay from the headend to theremote device experiencing the least delay is determined. The IMR isthen defined to be shorter than the first propagation delay and at leastas long as the difference between the two propagation delays. Thestarting point of the IMR is established by modifying the TS counteroutput of the headend. A modification value is added to the headend TScounter output. The modification value corresponds to a time intervalthat can be as long as the propagation delay from the headend to theremote having the shortest delay.

The invention described herein has the feature of calculating thedifference between the first and second propagation delays. Theinvention has the additional feature of determining the start of an IMRso as to take into account the propagation between the headend and theremote having the shortest propagation delay. The invention has theadvantage of allowing IMRs that require a minimum of upstream bandwidth.

The foregoing and other features and advantages of the invention will beapparent from the following, more particular description of a preferredembodiment of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 illustrates an example topology of a communications systemfeaturing a headend and a set of remote cable modems.

FIG. 2 illustrates a circuit that modifies a CMTS time base reference soas to alter the start point of an IMR, according to an embodiment of theinvention.

FIGS. 3A and 3B illustrate the synchronization process between a headendand a set of remote devices, according to DOCSIS.

FIG. 4 is a flowchart illustrating the process of defining an IMR thatreserves less bandwidth in the upstream, and the creation of a MAPmessage that conveys the IMR to remote devices, according to anembodiment of the invention.

FIG. 5 illustrates an example topology of a communications systemfeaturing a headend, an intermediate node, and a set of remote cablemodems.

FIGS. 6A and 6B illustrate the synchronization process between a headendand a set of remote devices, given a topology that includes anintermediate node.

FIG. 7 is a flowchart illustrating the process of defining an IMR thatreserves less bandwidth in the upstream and the creation of a MAPmessage that conveys the IMR to remote devices, given a topology thatincludes an intermediate node, according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention is now described withreference to the figures, where like reference numbers indicateidentical or functionally similar elements. Also in the figures, theleft-most digit of each reference number corresponds to the figure inwhich the reference number is first used. While specific configurationsand arrangements are discussed, it should be understood that this isdone for illustrative purposes only. A person skilled in the relevantart will recognize that other configurations and arrangements can beused without departing from the spirit and scope of the invention. Itwill be apparent to a person skilled in the relevant art that thisinvention can also be employed in a variety of other devices andapplications.

I. Overview

The invention described herein addresses the problem of how to conserveupstream bandwidth in a DOCSIS-based communications system, or similarsystem. An example of such a system is illustrated in FIG. 1. System 100includes a headend 105. Headend 105 may, for example, be a cable modemtermination system (CMTS). One such CMTS is the BCM93212, available fromBROADCOM Corporation, of Irvine, Calif. Downstream transmissions travelin direction 107; upstream transmissions travel in direction 108. Units110 through 170 represent remote devices. In a cable communicationssystem, each remote is a cable modem (CM). Note that the route ofcommunications between each cable modem and the headend 105 is slightlydifferent. For this and other reasons, the propagation delays betweenheadend 105 and each cable modem, respectively, can be different. Thepropagation delay between headend 105 and remote 110 (CM₁) is shown asT₁. In contrast, the propagation delay between headend 105 and remote170 (CM₇) is shown as T₂. In the conventional DOCSIS synchronizationprocess, the IMR is defined to accommodate the propagation delay betweenheadend 105 and the remote having the greatest propagation delay (here,remote 170). Recall that the IMR represents an interval in the upstreamduring which any of the associated remotes operating on the givenchannel can respond to the headend. The present invention shortens theIMR so that it only needs to accommodate the difference between theshortest and longest propagation delays, T₂−T₁. The resulting shorterIMR consumes less upstream bandwidth and therefore represents a moreefficient way to synchronize a headend with its associated remotedevices.

II. Apparatus

Definition of an IMR relies on the clock output at the headend. In anembodiment of the invention, a hardware implementation is used to createa 32-bit clock output, i.e., a counter, for an upstream channel. Suchcounters are known to persons of ordinary skill in the art.

FIG. 2 illustrates a hardware embodiment of the invention wherein theclock output for an upstream channel is modified by an offset. A 32-bitcounter 205 is clocked by a signal 210. In the embodiment shown in FIG.2, clock signal 210 has a frequency of 10.24 MHz. Counter 205 receives atime stamp load value 215 and is controlled by a load time stamp signal220. Time stamp load value 215 replaces any existing counter value ifsignal 220 so enables. Counter 205 produces counter output 225. Counteroutput 225, along with an offset value 230, are combined in adder 235,thereby modifying counter output 225. The offset 230, in an embodimentof the invention, corresponds to the propagation delay between theremote unit having the shortest propagation delay and the headend. In anembodiment of the invention, offset 230 is expressed in 98 nanosecondincrements. As described above, a clock signal can, in some systems, beinterpreted in terms of time units, or “ticks.” Ticks can be furtherorganized into larger units called minislots. The number of ticks perminislot can be defined at the discretion of the headend. The availableupstream bandwidth can therefore be viewed as a series of minislots.

The output of adder 235 is offset counter output 240. In the embodimentshown, output 240 enters bit shifter 245 and is right-shifted. Theshifting process is controlled by minislot count regulator signal 250.Each shift of one position, i.e., division by two, doubles the minislotsize (number of ticks per minislot). Hence the size of minislots can becontrolled.

The output of shifter 245 is divided into two signals, 260 (bit 0) and255 (the remaining 26 bits). Signal 260 is passed through inverter 270and sent to flip flop 275. The output of flip flop 275 is minislot clock285. The 26 bits of signal 255 are sent to 26 corresponding flip flops,shown collectively as flip flop 265. The output of flip flop 265 isminislot count 280. In the embodiment illustrated, flip flops 265 and275 are each driven by 20.48 MHz clocks 290 and 295, respectively. Also,in an embodiment of the invention, adder 235 is programmable, so thatoffset 230 can have different values as necessary.

This modification to counter output 225 serves to define the beginningof the IMR. Offsetting the output 225 serves to offset the valuerepresenting the point in the upstream at which any remote can respond.This value, now offset, is used in the MAP message. This delays thepoint in the upstream at which the remote can respond, and the allocatedIMR is effectively shorter than it would otherwise be. Hence lessupstream bandwidth is needed for the IMR.

III. Method

As described above, the invention described herein provides for thecreation of an IMR that conserves upstream bandwidth during thesynchronization process. The overall synchronization process isillustrated in FIGS. 3A and 3B. The process begins with step 305. Instep 310, the headend sends a synchronization message to all of itsassociated remote devices that will be operating on a given upstreamchannel. As discussed above, the synchronization message contains a timestamp that represents the current clock value at the headend. In step315, each such remote receives the synchronization message. In step 320,each remote locks on to the frequency of the headend clock, conveyed inthe synchronization message. In step 325, the remote compensates for anyknown delays by adjusting its local clock. In step 330, each remotedevice calculates a current local minislot count based on its localclock value.

In step 335, the headend sends a MAP message to all associated remotes,identifying the starting point, in the upstream, of an IMR. The startingpoint can be identified in terms of a specific minislot. In step 340,each remote receives the MAP message. In step 345, when a remote'sminislot count matches the minislot identified in the MAP message, theremote sends a burst transmission to the headend in response. A burstused in this context, for establishing synchronization, is known as aranging burst. In step 350, a burst demodulator in the headend comparesthe arrival time of the ranging burst with the expected arrival time ofthe burst. In step 355, the headend instructs the remote to adjust its(the remote's) local clock by the time difference. Once the remote doesso, the synchronization process concludes at step 360.

Note that DOCSIS also provides a procedure wherein contention by remotesfor an IMR is resolved. This involves the selection, by a remote, of arandom value between one and a specified initial backoff value. Theremote then sends a ranging burst in an IMR that corresponds to thisrandom value. If the ranging burst is not heard at the headend, anotherrandom value is selected by the remote, between one and a new backoffvalue. This process is described in detail in the DOCSIS 1.1specification, incorporated herein by reference in its entirety.

The step of creating and sending a MAP message that contains the IMRstarting point, step 335, is illustrated in greater detail in FIG. 4.The process begins at step 405. In step 410, the propagation delay T₁between the remote device experiencing the least propagation delay andthe headend's burst demodulator is determined. In step 415, thepropagation delay T₂ between the remote that experiences the greatestpropagation delay and the headend's burst demodulator is determined. Thedetermination of T₁ and T₂ can be accomplished in several ways. Forexample, linear distance can be measured, which can them divided by thespeed of light in the case of a fiber optic transmission medium.Alternatively, delays can be measured directly by conducting empiricaltests.

In step 420, the difference between the two delays (T₂−T₁) isdetermined. In step 425, an IMR is allocated, smaller than T₂ and atleast as large as the difference (T₂−T₁). In an embodiment of theinvention, the IMR is (T₂−T₁) in length. In the case where the IMR is(T₂−T₁) in length, the IMR starting point, as indicated in the MAPmessage, will be the current clock output plus T₁. Accordingly, in step427 the notion of time at the headend receiver is offset by T₁. In step430, a MAP message is created that expresses the starting point of thisIMR. In step 435, this MAP message is sent to the remote devices. Theprocess concludes at step 440.

In some contexts, synchronization may be performed on a periodic basisin order to maintain synchronicity between the headend and its remotes.At other times, system maintenance, disruption of operations, or otherexternal events could necessitate resynchronization. The process of FIG.4, however, is not necessarily repeated. The values T₁, T₂, and T₂−T₁should not change, so that in an embodiment of the invention, thisdifference only needs to be determined once, and the offset of theheadend is performed once.

IV. Alternative Topology

Other system topologies are possible, in addition to that of FIG. 1. Onesuch alternative topology is shown in FIG. 5. Downstream transmissionstravel from a headend 505 in direction 506; upstream transmissionstravel in direction 508. In this system, unlike that of FIG. 1, some ofthe functionality that would otherwise reside in a conventional CMTS(such as headend 105 of FIG. 1), is allocated to intermediate nodes.These intermediate nodes are illustrated in FIG. 5 as nodes 507 athrough 507 c. Physical layer processing, for example, can be handled inan intermediate node. This can include demodulation of upstreamtransmissions. In a burst communications system, therefore, anintermediate node can include one or more burst demodulators in additionto a burst receiver.

Units 510 through 550 represent remote devices. In a cablecommunications system, each remote is a cable modem (CM). The route ofcommunications between each cable modem and intermediate node 507 b isslightly different, so that the propagation delays between each cablemodem, respectively, and intermediate node 507 b and can be different.The propagation delay between remote 510 (CM₁) and intermediate node 507b is shown as T₁. In contrast, the propagation delay between remote 550(CM₅) and intermediate node 507 b is T₂.

In this topology, the IMR is defined to accommodate the propagationdelay between the remote having the greatest propagation delay (here,remote 550) and intermediate node 507 b. The IMR represents an intervalin the upstream during which any of the associated remotes operating onthe given channel can respond to the headend. As in the topology of FIG.1, the present invention shortens the IMR so that it only needs toaccommodate the difference between the shortest and longest propagationdelays, T₂−T₁. The resulting shorter IMR consumes less upstreambandwidth and therefore represents a more efficient way to synchronize aheadend and intermediate node with associated remote devices.

As described above, the invention described herein provides for thecreation of an IMR that conserves upstream bandwidth during thesynchronization process. The overall synchronization process isillustrated in in FIGS. 6A and 6B. The process begins with step 605. Instep 610, the headend sends a synchronization message to all of itsassociated remote devices that will be operating on a given upstreamchannel. As discussed above, the synchronization message contains a timestamp that represents the current clock value at the headend. In step615, each such remote receives the synchronization message. In step 620,each remote locks on to the frequency of the headend clock, conveyed inthe synchronization message. In step 625, each remote compensates forany known delays by adjusting its local clock. In step 630, each remotedevice calculates a current local minislot count based on its localclock value.

In step 635, the headend sends a MAP message to all associated remoteson a given upstream channel, identifying the starting point, in theupstream, of an IMR. The starting point can be identified in terms of aspecific minislot. In step each remote receives the MAP message, as doesthe intermediate node. In step 645, when a remote's minislot countmatches the minislot identified in the MAP message, the remote sends aranging burst to the intermediate node in response. In step 650, a burstdemodulator in the intermediate node compares the arrival time of theranging burst with the expected arrival time of the burst. In step 652,the difference between the arrival time and the expected arrival time isconveyed to the headend. In step 655, the headend instructs the remoteto adjust its (the remote's) local clock by the time difference. Oncethe remote does so, the synchronization process concludes at step 660.

The step of creating and sending a MAP message that contains the IMRstarting point, step 635, is illustrated in greater detail in FIG. 7.The process begins at step 705. In step 710, the propagation delaybetween the remote device experiencing the least propagation delay (CM510 in FIG. 5) and the intermediate node is determined. This delay isdenoted T₁ in FIG. 5. In step 715, the propagation delay between theremote that experiences the greatest propagation delay (CM 550 in FIG.5) and the intermediate node is determined (T₂). The determination of T₁and T₂ can be accomplished in several ways. For example, linear distancecan be measured, which can them divided by the speed of light in thecase of a fiber optic transmission medium. Alternatively, delays can bemeasured directly by conducting empirical tests.

In step 720, the difference between the two delays (T₂−T₁) isdetermined. In step 725, an IMR is allocated by the headend, smallerthan T₂ and at least as large as the difference T₂−T₁. In an embodimentof the invention, the IMR is (T₂−T₁) in length. In the case where theIMR is (T₂−T₁) in length, the IMR starting point, as indicated in theMAP message, will be the current clock output plus T₁. Accordingly, instep 727 the notion of time at the intermediate node's receiver isoffset by T₁. In step 730, a MAP message is created that expresses thestarting point of this IMR. In step 735, this MAP message is sent by theheadend to the remote devices. The process concludes at step 740.

In some contexts, synchronization may be performed on a periodic basisin order to maintain synchronicity between the headend and its remotes.At other times, system maintenance, disruption of operations, or otherexternal events could necessitate resynchronization. The process of FIG.7, however, is not necessarily repeated. The values T₁, T₂, and T₂−T₁should not change, so that in an embodiment of the invention, thisdifference only needs to be determined once, and the offset of theintermediate node's receiver is performed once.

V. Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art that various changes in detail can be made thereinwithout departing from the spirit and scope of the invention. Thus thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. A method of allocating an initial maintenance region for an upstreamchannel in a communications system, wherein the communications systemcomprises a headend and at least one remote device associated with thechannel, said method comprising the steps of: a. determining apropagation delay T₂ from a remote device experiencing the greatestdelay of all remote devices associated with the channel, to ademodulator that demodulates upstream transmissions; b. determining apropagation delay T₁ from a remote device experiencing the least delayof all remote devices associated with the channel, to the demodulator;c. defining the size of the initial maintenance region to be less thanT₂, and at least as great as the difference between the propagationdelays, T₂−T₁.
 2. The method of claim 1, further comprising the step of:d. creating a MAP that expresses the starting point of the initialmaintenance region.
 3. The method of claim 1, further comprising thestep of: d. sending a MAP to the at least one remote device wherein theMAP expresses the starting point of the initial maintenance region. 4.The method of claim 1, wherein step c comprises the steps of: i) adding,to a clock output associated with the channel, a programmable offsetthat is less than or equal to T₁, to form an offset clock output; ii)translating the offset clock output to a corresponding offset minislotcount; and iii) defining the starting point of the initial maintenanceregion according to the offset minislot count.
 5. The method of step 4,wherein the programmable offset is defined in terms of periods of a10.24 MHz signal.
 6. The method of claim 1, wherein the remote devicesare cable modems.
 7. The method of claim 1, wherein the headend is acable modem termination system.
 8. The method of claim 1, wherein thedemodulator is incorporated in the headend.
 9. The method of claim 1,wherein the demodulator is incorporated in an intermediate node.
 10. Acommunications system in which an initial maintenance region isallocated in a communications channel for purposes of allowingsynchronization between communicating entities, the system comprising: aheadend for delivery and receipt of information; at least one remotedevice that is associated with said headend and that transmitsinformation to said headend via an upstream channel; clock circuitry insaid headend for generating a clock output that marks regular intervalsin said upstream channel; an adder in said headend for adding an offsetto the clock output, to offset a starting point of the initialmaintenance region.
 11. The system of claim 10, wherein said adder isprogrammable.
 12. The system of claim 10, wherein said offset representsa time interval less than or equal to a propagation delay between aremote device having the shortest propagation delay of all remotedevices associated with the upstream channel and said demodulator. 13.The system of claim 12, wherein said headend comprises said demodulator.14. The system of claim 12, further comprising an intermediate nodebetween said headend and said at least one remote device, wherein saidintermediate node comprises said demodulator.